Help with texturing a test model
You have not assigned the image texture to the material. That's the reason why it's no shown on the cube.
In the material properties, click the yellow dot right next to Base Color and select Image Texture. Then click the image icon with a little arrow next to it that just appeared. In the popup select your image. Done.
Alternatively, go to the Shader Editor. Add an Image Texture node, select the image, and connect the color output of the Image Texture node to the Base Color input of the Principled BSDF.